Many types of lifting apparatus employing a scissors mechanism are known. In most cases the scissors mechanism is operated by a hydraulic or pneumatic piston or by a screw drive or equivalent. These devices involve a difficulty in that a much greater amount of power is required at the beginning of the lifting operation than at the end thereof. As a result relatively large power capabilities must be provided, with attendant capital and operating costs.
Lifting devices are also known in which the scissors mechanism is operated by a cam which forces the scissors apart and wherein a curved cam path is provided on the scissors mechanism. Reference is made in this regard to German Pat. Nos. 604,156 and 1,175,852 which show such a structure. Reference is also made to U.S. Pat. No. 3,785,462 which shows a similar structure without a curved cam path.
Furthermore, many types of wheeled vehicles are known. One general type of such vehicle which is in common use in materials handling is commonly described as a dolly and is characterized in that at least some of its wheels comprise castors. For the purposes of this discussion, castors may be understood as comprising a wheel mounted on a mounting element, which element is in turn rotatably mounted about an axis which is non-intersecting with the axis of rotation of the wheel.
Dollies of the conventional type employing castors have a number of significant disadvantages. Most important among these is the difficulty in maneuvering the dolly, particularly when heavy loads are involved. This difficulty arises due to the tendency of castors to become locked in an orientation wherein they cannot roll in a desired direction. This tendency renders it nearly impossible in practice to move a conventional dolly back and forth along a straight line without spurious sideways travel.
Another significant disadvantage of conventional dollies is their relatively large turning radius. This characteristic limits the use of dollies in industrial and other applications and adds to space requirements in industrial plant design.
An additional disadvantage of conventional dollies is the relatively large amount of force required to move them to a desired location. As a result of the large force requirements, mechanized means must often be employed for maneuvering the dollies. Precise positioning of the dollies, which is required in many applications, is particularly difficult to achieve using mechanized apparatus. Furthermore, maneuvering conventional dollies normally requires both a pushing and a pulling capability.
An additional significant difficulty encountered with conventional dollies carrying heavy loads is wheel wear due to sliding motion of wheels during turning operations. The large amount of wear requires frequent wheel changes with resulting significant downtime of the equipment.
Castors are known having more than one axis of rotation. Reference is made in this connection to U.S. Pat. No. 1,797,830 which illustrates an automobile turntable employing such a castor and to U.S. Pat. No. 474,576 which illustrates such a castor designed to be attached to a furniture leg. The castors described in these references are all characterized in that they comprise a plate and roller assembly for vertically supporting the wheel. This arrangement greatly increases the frictional forces encountered in the operation of the castor and thus increases the force required for moving a load supported on the castor.